1. Field of the Invention
This invention relates to graphite materials with chemically-formed silicon carbide on its surface, which can be used as mechanical seals and a process for producing the same materials.
2. Description of the Prior Art
As compared with bare graphite products, SiC-coated graphite has superior properties such as higher strength, higher hardness, higher abrasion resistance, and higher oxidation resistance. Therefore, it is one of the best useful materials for use in mechanical seals.
There are three major methods for the formation of SiC on graphite: (i) chemical vapor deposition employing organosilicon compounds such as trimethylsilane, (ii) infiltration-reactions employing Si liquid or gas; and (iii) gas-solid reactions employing SiO gas.
U.S. Pat. No. 3,591,243 discloses the bearing elements which consist of coherent pyrolytic silicon carbide formed by Si-carring gas through chemical vapor deposition.
U.K. Patent Specification No. 1,118,056 provides a process for cladding or coating a carbonaceous substrate with silicon carbide. This is accomplished by melting a thin layer of highly pure silicon on carbonaceous substrates, and then by depositing homogeneous silicon carbide over the surface of the substrate.
U.S. Pat. No. 4,221,831 employs molten silicon (Si) as a siliconization agent for porous carbon blank. Impregnation of the carbon blank with molten silicon starts at a temperature ranging from 2100.degree. to 2200.degree. C. for 3 to 5 minutes, followed by transformation to silicon carbide at a temperature ranging from 1800.degree. to 2050.degree. C. for 30 to 40 minutes.
Japanese Patent Publication No. (Sho) 38-16,106 employs a mixture of silicon powder (Si) and silica (SiO.sub.2) as a silicon monoxide source for the conversion of graphite to silicon carbide. SiC-coated layer of 0.2-0.5 mm thickness is achieved at a temperature of 1800.degree. to 2300.degree. C. for 150 minutes.
Japanese Laid-Open Patent Publication No. (Sho) 59-3,907 employs a mixture of silica (SiO.sub.2) and carbon black (C) as a silicon monoxide source for the conversion of graphite to silicon carbide.
U.S. Pat. No. 3,634,116 teaches silicon monoxide vapor, which is produced by heating silicon powder (Si) and silica (SiO.sub.2) to a temperature of at least 1400.degree. C. and reacts with graphite particles of the substrate to form individual silicon carbide casings on the graphite particles.
U.K. Patent Specification No. 2,122,179 concerns a method for forming SiC by the reaction of SiO gas with graphite and a carbon substrate. In this method, SiO gas is generated by reacting SiO.sub.2 powder with hydrogen gas within a temperature ranging from 1425.degree. to 1600.degree. C. The layer of SiC has 1.5 mm thickness and a density of 2.6 g/cm.sup.3.
Japanese Laid-Open Patent Publication No. (Sho) 54-28,311 discloses a process for making a uniform, SiC-coated graphite. It employs a graphite tube furnace which has a temperature gradient ranging from 300.degree. C. at the entrance to 2000.degree. C. at the center. A graphite crucible having two separated zones is used to generate silicon monoxide and to siliconize the graphite substrate. Silicon monoxide is generated by reacting a mixture of silica or silicon compound and carbon. The yielded products have a uniform SiC-coated layer of about 0.8 mm thickness.